In a mobile communication system employing the LTE (Long Term Evolution) system, which is specified in the 3GPP, a handover source radio base station S-eNB is configured to perform the following operation, when determining that a mobile station UE is to perform a handover (X2 handover) from a cell controlled by the handover source radio base station S-eNB to a cell controlled by a handover destination radio base station T-eNB. Here, the handover source radio base station S-eNB transfers consecutive uplink data (accumulated in a receiving buffer) to a serving gateway apparatus S-GW, by using a U-plane resource (GTP (GPRS Tunneling Protocol)-U tunnel) allocated between the handover source radio base station S-eNB and the serving gateway apparatus S-GW, and also transfers non-consecutive uplink data (accumulated in the receiving buffer) to the handover destination radio base station T-eNB. The consecutive uplink data refers to uplink data that are received by the handover source radio base station S-eNB from the mobile station UE with their sequence numbers being consecutive (In-sequence). The non-consecutive uplink data refers to uplink data that are received by the handover source radio base station S-eNB from the mobile station UE with their sequence numbers being not consecutive (Out-sequence).
In addition, in the aforementioned mobile communication system, the handover source radio base station S-eNB is configured to notify, to the handover destination radio base station T-eNB, a transfer status of the non-consecutive uplink data to the handover destination radio base station T-eNB by using a C-plane resource (specifically, to transmit an “SN Status Transfer” message).
Meanwhile, in the aforementioned mobile communication system, the handover source radio base station S-eNB may be configured to perform the following operation, when determining that a mobile station UE is to perform a handover (S1 handover) from a cell controlled by the handover source radio base station S-eNB to a cell controlled by a handover destination radio base station T-eNB. Here, the handover source radio base station S-eNB uses “Indirect Forwarding” to transfer, to the serving gateway apparatus S-GW, consecutive uplink data (accumulated in the receiving buffer) received from the mobile station UE with their sequence numbers being consecutive (In-sequence), by using a U-plane resource (GTP-U tunnel) allocated between the handover source radio base station S-eNB and the serving gateway apparatus S-GW. Meanwhile, the handover source radio base station S-eNB discards non-consecutive uplink data (accumulated in the receiving buffer) received from the mobile station UE with their sequence numbers being not consecutive (Out-sequence).
Moreover, in the aforementioned mobile communication system, the handover source radio base station S-eNB may be configured to perform the following operation, when determining that a mobile station UE is to perform a handover (S1 handover) from a cell controlled by the handover source radio base station S-eNB to a cell controlled by the handover destination radio base station T-eNB. Here, the handover source radio base station S-eNB uses “Direct Forwarding” to transfer, to the handover destination radio base station T-eNB, consecutive uplink data (accumulated in the receiving buffer) received from the mobile station UE with their sequence numbers being consecutive (In-sequence), by using a U-plane resource (GTP-U tunnel) allocated between the handover source radio base station S-eNB and the handover destination radio base station T-eNB. Meanwhile, the handover source radio base station S-eNB discards non-consecutive uplink data (accumulated in the receiving buffer) received from the mobile station UE with their sequence numbers being not consecutive (Out-sequence).
Further, in the aforementioned mobile communication system, the handover source radio base station S-eNB is configured to perform the following operation, when determining that the mobile station UE is to perform a handover (S1 handover) from a cell controlled by the handover source radio base station S-eNB to a cell controlled by the handover destination radio base station T-eNB. Here, the handover source radio base station S-eNB transfers downlink data to the handover destination radio base station T-eNB, the downlink data being downlink data which are among the downlink data received from the serving gateway apparatus S-GW and whose transmission to the mobile station UE is not completed yet (downlink data for which ACK is not received).
However, in the conventional mobile communication system employing the LTE system, there is a problem that, when a mobile station UE performs an X2 handover and an S1 handover, the serving gateway apparatus S-GW cannot know how long the serving gateway apparatus S-GW needs to hold the U-plane resource (GTP-U tunnel), because the serving gateway apparatus S-GW (or handover destination radio base station T-eNB) cannot detect completion of the transfer of the consecutive uplink data to the serving gateway apparatus S-GW by the handover source radio base station S-eNB.
In addition, there is another problem in the conventional mobile communication system employing the LTE system. The problem is that, when a mobile station UE performs an S1 handover, the handover destination radio base station T-eNB cannot perform sequence control between the downlink data received from the serving gateway apparatus S-GW and the downlink data received from the handover source radio base station S-eNB, because the handover destination radio base station T-eNB cannot detect completion of the transfer of the downlink data to the handover destination radio base station T-eNB by the handover source radio base station S-eNB.